The intrathecal administration of the anticancer agent mercaptopurine (i.e. directly into the cerebrospinal fluid, CSF, of the central nervous system) may provide an effective method for the treatment of acute lymphocytic leukemia (ALL). Such treatment requires careful control of drug levels in the CSF. With high speed digital computers it may be possible to use a sophisticated model of mercaptopurine kinetics in conjunction with a mathematical algorithm for dosage selection to rapidly and effectively control the CSF concentration of mercaptopurine. Toward this goal the metabolism of 6-MP has been investigated in monkeys following intrathecal and intravenous administration of mercaptopurine. The major findings of the research have been the development of a physiological-pharmacokinetic model of mercaptopurine kinetics in the CSF and the development of statistical methdos to determine subject specific model parameters. The latter result permits 'real time' evaluation of the appropriate dosage amount and duration of administration. The methodology is currently being tested on monkeys.